request.getSession(true/false)的区别

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javax.servlet.http.HttpServletRequest接口有两个方法：getSession(boolean)和getSession()。
具体什么区别，跟踪源码分析下，先摆出结论：
request.getSession(true)：获取session,如果session不存在，就新建一个。
reqeust.getSession(false)获取session,如果session不存在，则返回null。
Debug时，查看HttpServletRequest接口的实现类为RequestFacade。

使用Idea查看RequestFacade的代码实现，可以看出是通过Facade外观模式对org.apache.catalina.connector.Request进行了封装。
继续看getSession()的源码，其实是调用了getSession(true)。具体是调用了request.getSession(create)。

@Override
public HttpSession getSession(boolean create) {

    if (request == null) {
        throw new IllegalStateException(
                        sm.getString("requestFacade.nullRequest"));
    }

    if (SecurityUtil.isPackageProtectionEnabled()){
        return AccessController.
            doPrivileged(new GetSessionPrivilegedAction(create));
    } else {
        return request.getSession(create);
    }
}

@Override
public HttpSession getSession() {

    if (request == null) {
        throw new IllegalStateException(
                        sm.getString("requestFacade.nullRequest"));
    }
    // 直接调用getSession(true)
    return getSession(true);
}

进入到Request.getSession(boolean)，根据注释看出，create为true时，如果HttpSession不存在，会创建一个新的HttpSession。

    /**
     * @return the session associated with this Request, creating one
     * if necessary and requested.
     *
     * @param create Create a new session if one does not exist
     */
    @Override
    public HttpSession getSession(boolean create) {
        Session session = doGetSession(create);
        if (session == null) {
            return null;
        }

        return session.getSession();
    }

继续进入到doGetSession(boolean create)方法，继续分析。

    protected Session doGetSession(boolean create) {

        // There cannot be a session if no context has been assigned yet
        Context context = getContext();
        if (context == null) {
            return (null);
        }

        // Return the current session if it exists and is valid
        // 如果当前session存在且有效，返回当前session
        if ((session != null) && !session.isValid()) {
            session = null;
        }
        if (session != null) {
            return (session);
        }

        // Return the requested session if it exists and is valid
        // 这里有读写锁控制并发
        Manager manager = context.getManager();
        if (manager == null) {
            return (null);      // Sessions are not supported
        }
        if (requestedSessionId != null) {
            try {
                session = manager.findSession(requestedSessionId);
            } catch (IOException e) {
                session = null;
            }
            if ((session != null) && !session.isValid()) {
                session = null;
            }
            if (session != null) {
                session.access();
                return (session);
            }
        }

        // Create a new session if requested and the response is not committed
        // create为false时，返回null；create为true时创建一个新的session
        if (!create) {
            return (null);
        }
        if (response != null
                && context.getServletContext()
                        .getEffectiveSessionTrackingModes()
                        .contains(SessionTrackingMode.COOKIE)
                && response.getResponse().isCommitted()) {
            throw new IllegalStateException(
                    sm.getString("coyoteRequest.sessionCreateCommitted"));
        }

        // Re-use session IDs provided by the client in very limited
        // circumstances.
        String sessionId = getRequestedSessionId();
        if (requestedSessionSSL) {
            // If the session ID has been obtained from the SSL handshake then
            // use it.
        } else if (("/".equals(context.getSessionCookiePath())
                && isRequestedSessionIdFromCookie())) {
            /* This is the common(ish) use case: using the same session ID with
             * multiple web applications on the same host. Typically this is
             * used by Portlet implementations. It only works if sessions are
             * tracked via cookies. The cookie must have a path of "/" else it
             * won't be provided for requests to all web applications.
             *
             * Any session ID provided by the client should be for a session
             * that already exists somewhere on the host. Check if the context
             * is configured for this to be confirmed.
             */
            if (context.getValidateClientProvidedNewSessionId()) {
                boolean found = false;
                for (Container container : getHost().findChildren()) {
                    Manager m = ((Context) container).getManager();
                    if (m != null) {
                        try {
                            if (m.findSession(sessionId) != null) {
                                found = true;
                                break;
                            }
                        } catch (IOException e) {
                            // Ignore. Problems with this manager will be
                            // handled elsewhere.
                        }
                    }
                }
                if (!found) {
                    sessionId = null;
                }
            }
        } else {
            sessionId = null;
        }
        session = manager.createSession(sessionId);

        // Creating a new session cookie based on that session
        if (session != null
                && context.getServletContext()
                        .getEffectiveSessionTrackingModes()
                        .contains(SessionTrackingMode.COOKIE)) {
            Cookie cookie =
                ApplicationSessionCookieConfig.createSessionCookie(
                        context, session.getIdInternal(), isSecure());

            response.addSessionCookieInternal(cookie);
        }

        if (session == null) {
            return null;
        }

        session.access();
        return session;
    }

StandardContext跟session没有啥关系，就是学习下StandardContext的源码及ReentrantReadWriteLock。

@Override
    public Manager getManager() {
        Lock readLock = managerLock.readLock();
        readLock.lock();
        try {
            return manager;
        } finally {
            readLock.unlock();
        }
    }


    @Override
    public void setManager(Manager manager) {

        Lock writeLock = managerLock.writeLock();
        writeLock.lock();
        Manager oldManager = null;
        try {
            // Change components if necessary
            oldManager = this.manager;
            if (oldManager == manager)
                return;
            this.manager = manager;

            // Stop the old component if necessary
            if (oldManager instanceof Lifecycle) {
                try {
                    ((Lifecycle) oldManager).stop();
                    ((Lifecycle) oldManager).destroy();
                } catch (LifecycleException e) {
                    log.error("StandardContext.setManager: stop-destroy: ", e);
                }
            }

            // Start the new component if necessary
            if (manager != null) {
                manager.setContext(this);
            }
            if (getState().isAvailable() && manager instanceof Lifecycle) {
                try {
                    ((Lifecycle) manager).start();
                } catch (LifecycleException e) {
                    log.error("StandardContext.setManager: start: ", e);
                }
            }
        } finally {
            writeLock.unlock();
        }

        // Report this property change to interested listeners
        support.firePropertyChange("manager", oldManager, manager);
    }

结论：
request.getSession(true)：获取session,如果session不存在，就新建一个。
reqeust.getSession(false)获取session,如果session不存在，则返回null。